I setup X3 with the demo content and while I was going through the examples I noticed that most of the thumbnails only showed text. The following content was displayed:
Trying to access array offset on value of type null
And a reference to app/parsers/slir/slirrequest.class.php on line 526. I googled the error which led me to the assumption that the error was PHP 7.4 related (because of this: https://github.com/composer/composer/issues/8324). I switched from 7.4 to 7.3 and it worked.